In recent years, there is a need for line pipes used for transporting natural gas and crude oil to have higher strength, in order to improve transport efficiency by high pressure operation. Specifically, the line pipes are required to have high deformability so that the occurrence of cracks can be prevented even when the line pipes are largely deformed by ice gouging or ground deformation. For example, in pipe lines constructed in seismic regions or on the seabed in cold climates where ice gouging occurs, line pipes with high uniform elongation and a low yield ratio of 90% or less are required.
Welded steel pipes such as UOE steel pipes and ERW steel pipes are used for line pipes. Such a welded steel pipe is manufactured by subjecting a steel plate/sheet to cold forming into a pipe shape, welding the seam, and then generally subjecting the outer surface of the steel pipe to coating treatment from the viewpoint of corrosion protection. However, a strain age hardening phenomenon occurs due to work strain imposed during the manufacturing of the pipe and heating during the coating treatment, and this causes an increase in yield stress, so that a problem arises in that the yield ratio of the steel pipe is larger than the yield ratio of the steel plate.
In line pipes used to transport natural gas and crude oil containing hydrogen sulfide, hydrogen generated by the reaction of the hydrogen sulfide and the steel enters the steel, and this may cause cracking. Therefore, such line pipes are required to have hydrogen-induced cracking resistance (HIC resistance) in addition to strength, high uniform elongation, low yield ratio, and strain aging resistance.
One known effective method for achieving a low yield ratio and high uniform elongation is to produce a steel material having a metallographic structure in which hard phases such as bainite and martensite are properly dispersed in a soft phase such as ferrite. One known effective method for preventing hydrogen-induced cracking is to reduce P etc. having a strong tendency to segregation. As gas fields are increasingly developed, a wide variety of sour environment (pH, hydrogen sulfide partial pressure) is increasing, and attention is given to a mildly sour environment (a wet hydrogen sulfide environment). In an environment having a relatively low acidity with a pH of 5 or more, i.e., a so-called mildly sour environment, it is known that addition of Cu to steel to form a protective coating on the steel material is effective in suppressing penetration of hydrogen into the steel.
Patent Literature 1 discloses a manufacturing method for obtaining a structure in which a hard phase is properly dispersed in a soft phase. This manufacturing method includes a heat treatment method in which quenching from a two-phase region of ferrite and austenite is performed between quenching and tempering.
Patent Literature 2 discloses a technique for achieving a low yield ratio without performing the complicated heat treatment disclosed in Patent Literature 1. In this method, rolling of a steel material is finished at a temperature equal to or higher than the Ar3 temperature, and then the rate of accelerated cooling and cooling stop temperature are controlled to obtain a two-phase structure of acicular ferrite and martensite, whereby a low yield ratio is achieved.
As for the strain aging resistance, Patent Literatures 3 and 4, for example, disclose low-yield ratio, high-strength and high-toughness steel pipes having superior strain aging resistance and methods for manufacturing the steel pipes. Specifically, a fine precipitate of a composite carbide containing Ti and Mo or a fine precipitate of a composite carbide containing at least two of Ti, Nb, and V is utilized.
Patent Literature 5 discloses a method for achieving a low yield ratio, high strength, high uniform elongation, superior strain aging resistance, and API 5L X70 or lower without greatly increasing the amounts of alloy elements added to a steel material. In this method, reheating is performed immediately after accelerated cooling, and a three-phase structure including bainite, polygonal ferrite, and martensite-austenite constituent (MA) is thereby obtained.
Patent Literature 6 discloses a method for achieving HIC resistance in a steel material with API 5L X65 or higher and having a two-phase structure of ferrite and bainite. In this method, the difference in hardness between the ferrite and bainite is reduced.